Glucose-dependent insulinotropic polypeptide (“GIP”, also known as “gastric inhibitory polypeptide”; SEQ ID NO:1) is a 42-residue peptide secreted by enteroendorine K-cells of the small intestine into the bloodstream in response to oral nutrient ingestion. GIP inhibits the secretion of gastric acid, and it has been shown to be a potent stimulant for the secretion of insulin from pancreatic beta cells after oral glucose ingestion (the “incretin effect”) (Creutzfeldt, W., et al., 1979, Diabetologia, 16:75-85).
Insulin release induced by the ingestion of glucose and other nutrients is due to both hormonal and neural factors (Creutzfeldt, W., et al., 1985, Diabetologia, 28:565-573). Several gastrointestinal regulatory peptides have been proposed as incretins, and among these candidates, only GIP and glucagon-like peptide 1 (“GLP-1”) appear to fulfill the requirements to be considered physiological stimulants of postprandial insulin release (Nauck, et al., 1989, J. Clin. Endorinol. Metab., 69:654-662). It has been shown that the combined effects of GIP and GLP-1 are sufficient to explain the full incretin effect of the enteroinsular axis (Fehmann, H. C., et al., 1989, FEBS Lett., 252:109-112).
As is well known to those skilled in the art, the known and potential uses of GIP are varied and multitudinous. Thus, the administration of the compounds of this invention for purposes of eliciting an agonist effect can have the same effects and uses as GIP itself. These varied uses of GIP may be summarized as follows: treating a disease selected from the group consisting of type 1 diabetes, type 2 diabetes (Visboll, T., 2004, Dan. Med. Bull., 51:364-70), insulin resistance (WO 2005/082928), obesity (Green, B. D., et al., 2004, Current Pharmaceutical Design, 10:3651-3662), metabolic disorder (Gault, V. A., et al., 2003, Biochem. Biophys. Res. Commun., 308:207-213), central nervous system disease, neurodegenerative disease, congestive heart failure, hypoglycemia, and disorders wherein the reduction of food intake and weight loss are desired. In pancreatic islets, GIP not only enhances insulin secretion acutely, but it also stimulates insulin production through enhancement of proinsulin transcription and translation (Wang, et al., 1996, Mol. Cell. Endocrinol., 116:81-87) and enhances the growth and survival of pancreatic beta cells (Trumper, et al., 2003 Diabetes, 52:741-750). In addition to effects on the pancreas to enhance insulin secretion, GIP also has effects on insulin target tissues directly to lower plasma glucose: enhancement of glucose uptake in adipose (Eckel, et al., 1979, Diabetes, 28:1141-1142) and muscle (O'Harte, et al., 1998, J. Endocrinol., 156:237-243), and inhibition of hepatic glucose production (Elahi, D., et al., 1986, Can. J. Physiol. Pharmacol., 65:A18).
In addition, a GIP receptor antagonist in accordance with the present invention inhibits, blocks or reduces glucose absorption from the intestine of an animal. In accordance with this observation, therapeutic compositions containing GIP antagonists may be used in patients with non-insulin dependent diabetes mellitus to improve tolerance to oral glucose in mammals, such as humans, to prevent, inhibit or reduce obesity by inhibiting, blocking or reducing glucose absorption from the intestine of the mammal.
The use of unmodified GIP as a therapeutic, however, is limited by the short in vivo half-life of about 2 minutes (Said and Mutt, 1970, Science, 169:1217-1218). In serum, both incretins, GIP and GLP-1, are degraded by dipeptidyl peptidase IV (“DPPIV”). Improving the stability of GIP to proteolysis not only maintains the activity of GIP at its receptor but, more importantly, prevents the production of GIP fragments, some of which act as GIP receptor antagonists (Gault, et al., 2002, J. Endocrinol., 175:525-533). Reported modifications have included protection of the N-terminus of GIP from proteolysis by DPPIV through modification of the N-terminal tyrosine (O'Harte, et al., 2002, Diabetologia, 45:1281-1291), mutation of the alanine at position 2 (Hinke, et al., 2002, Diabetes, 51:656-661), mutation of glutamic acid at position 3 (Gault, et al., 2003, Biochem. Biophys. Res. Commun., 308:207-213), and mutation of alanine at position 13 (Gault, et al., 2003, Cell Biol. International, 27:41-46),
The following patent applications have been filed related to the effects of GIP analogues on the function of various target organs and their potential use as therapeutic agents:
PCT publication WO 00/58360 discloses peptidyl analogues of GIP which stimulate the release of insulin. In particular, this application discloses specific peptidyl analogues comprising at least 15 amino acid residues from the N-terminal end of GIP(1-42), e.g., an analogue of GIP containing exactly one amino acid substitution or modification at positions 1, 2 and 3, such as [Pro3]GIP(1-42).
PCT publication WO 98/24464 discloses an antagonist of GIP consisting essentially of a 24-amino acid polypeptide corresponding to positions 7-30 of the sequence of GIP, a method of treating non-insulin dependent diabetes mellitus and a method of improving glucose tolerance in a non-insulin dependent diabetes mellitus patient.
PCT publication WO 03/082898 discloses C-terminal truncated fragments and N-terminal modified analogues of GIP, as well as various GIP analogues with a reduced peptide bond or alterations of the amino acids close to the DPPIV-specific cleavage site. This application further discloses analogues with different linkers between potential receptor binding sites of GIP. The compounds of this application are alleged to be useful in treating GIP-receptor mediated conditions, such as non-insulin dependent diabetes mellitus and obesity.
There exists a need for improved analogues of GIP, which are stable in formulation and have long plasma half-life in vivo resulting from decreased susceptibility to proteolysis and decreased clearance while maintaining binding affinity to a GIP receptor to elicit respective agonistic or antagonistic effects. Moreover, among other therapeutic effects of the compounds of the present invention as illustrated herein, tighter control of plasma glucose levels may prevent long-term diabetic complications, thereby providing an improved quality of life for patients.